Although state and local mandatory helmet laws have become widespread in the US, there is almost no information on whether such regulations actually decrease injuries from bicycle-related accidents. The first aim is to use pooled, annual, hospital-level data from the National Electronic Injury Surveillance System (NEISS) to assess whether mandatory helmet laws implemented in states and localities in the United States between 1991 and 2007 were effective in reducing injuries among youth. We focus on bicycle-related traumatic head injuries among youth aged 5 to 18 that require emergency room treatment. Since helmet laws vary widely in the age groups targeted (e.g. under 12, under 18), we also plan to assess the relative effectiveness of helmet laws that pertain to different age groups. The second aim is to use data from the NEISS to assess whether helmet laws have had spillover effects on age groups and on other recreational activities not specifically targeted by the laws. Although the majority of bicycle helmet laws pertain to young children, the laws may encourage older children and adults to wear helmets, and thus may reduce head injuries in those groups as well. In addition, children who are required to wear helmets while bicycling may be more likely to wear helmets while participating in similar sports (e.g., ATV riding), potentially reducing head injuries associated with those sports as well. To address both these aims, we use regression analyses to link hospital-level injury rates to state and local level helmet laws, controlling for a range of other factors that affect injury rates. In sensitivity analyses, we will investigate whether helmet laws are associated with increases in helmet usage among youth, and whether there are effects of laws on risky cycling behaviors and ridership. PUBLIC HEALTH RELEVANCE: This study would be the first to consider the effects of bicycle helmet laws on bicycle-related injuries that are reported by hospital emergency departments. The results from this study will provide policymakers with new, important information not only about the effectiveness of existing helmet laws, but also about how these laws can be designed to maximize their preventive benefits.